Vol. 65.] GLACIAL EROSION IN NORTH WALES. ' 287 



outline, and yet perceived that the changes had progressed so far 

 that they could not be traced : 



'To speak strictly, the original figure influences all the subsequent; 



but the farther removed from it in point of time, the less is that influence ; 

 so that for the purpose of such approximations as suit the im- 

 perfection of our knowledge, the consideration of the original figure may be 

 wholly left out.' — 'Illustrations of the Huttonian Theory of the Earth' 1802, 

 p. 371. 



It suffices, therefore, for the present to say that the mountains of 

 Wales, as we see them to-day, are the deep-worn remnants of 

 mountains that were once much greater than they are now : upon 

 this there is general agreement. 



When, however, we come to estimate the share that different 

 erosive agencies have had in shaping the Welsh mountains, opinions 

 differ widely. An earlier generation of geologists, with Eamsay as 

 the leader and Mackintosh (1869) as the extremest exponent, 

 attributed a great amount of erosion to the sea ; Ramsay thought 

 that a ' plain of marine denudation ' had been worn and cut across 

 the pre-existent mountains of deformation ; that after the uplift of 

 this plain, subaerial erosion excavated valleys in it : that glacial 

 erosion at a still later period 



' somewhat deepened, widened, smoothed, and striated the minor outlines of the 

 mountains and valleys, and excavated many rock-bound lake-basins, but on a 

 grand scale did not effect any great changes on the pre-existing larger contours 

 of the country ' (1881, p. 322) ; 



and that the effects of erosion in post-Glacial time have been small 

 ' simply from lack of time.' The chief post-Glacial changes are a 

 small wasting of cliffs to form talus-slopes, and the erosion of small 

 gorges and the deposition of small fans and deltas by the larger 

 streams. The analogy already pointed out between Wales and 

 the Scottish Highlands may be here extended ; for, since the 

 appearance of Ramsay's essay on denudation in Wales, the Highland 

 glens have been repeatedly described as due chiefly to normal 

 erosion acting on an uplifted plain of marine denudation, and as 

 only modified in a subordinate way by later glacial erosion. 



A later generation of geologists seems gradually to have given up 

 marine erosion in favour of 'subaerial' erosion, — called normal 

 erosion in this essay — as the agency by which the great ancient 

 mountains of Scotland and Wales were for the most part worn 

 down : marine erosion being nowadays appealed to — if appealed to 

 at all — only in order to give the last touches in the work of 

 planation. Sir Archibald Geikie's essay ' On Modern Denudation ' 

 (1868) appears to have played an important part in causing this 

 change of opinion in Great Britain ; nevertheless, explicit re- 

 cognition of the sufficiency of subaerial erosion to reduce a 

 mountain-mass to a peneplain without supplement by marine 

 erosion is seldom found in British geological literature ; and still 

 more rarely does one find there a deliberate and thoroughgoing 

 analysis of the consequences of this simple idea. Here one may 



